1. Field of the Invention
This invention relates generally to class-D amplifiers. More particularly, the invention relates to class-D transconductance audio amplifiers.
2. Related Art
Conventional class-D amplifiers operate as voltage-controlled voltage sources and are often utilized in audio electronics to drive loudspeakers. Class-D amplifiers are also known as “switched mode” amplifiers because they operate by switching transistors, such as field effect transistors (FETs), at a carrier frequency to create a switched carrier signal. The switched carrier signal is typically a pulse width modulated (PWM) signal in the 300 kilohertz (kHz) to 500 kHz range that varies depending upon the amplitude of the input signal. Audio input signals typically in the range of 20 Hertz (Hz) to 20 kilohertz (kHz) are propagated to the loudspeakers via the switched carrier signal. A feedback path provides feedback to the amplifier input to reduce both distortion and output impedance. Low-pass filtering is used to filter the carrier and reconstruct the propagated audio signal at the load.
FIG. 1 is a block diagram of a conventional class-D amplifier 100. An audio input signal having a voltage Ue is provided to the amplifier 100 at input terminal 102 relative to a terminal 104 at ground potential. An input resistor R2 106 is connected between the input terminal 102 and an inverting input node 108 of an operational amplifier 110. The operational amplifier 110 is configured as an integrator with a feedback network 114 and the input resistor 106. Details on suitable components for the feedback network 114 may be found within a technical reference on class-D amplifiers. For purposes of this description, the feedback network 114 may include two capacitors in series in the feedback path with a resistor connected between the interconnection of the series capacitors and ground.
The output of the inverting amplifier 110 at node 112 is connected to a power switch 116 that switches depending upon the sign of the audio signal to drive a positive or negative voltage to a node 118. The power switch 116 includes a gate driver and metal oxide semiconductor FET (MosFET) power transistors (not shown) to provide voltage switching and current supply capabilities. The voltage at node 118 is fed back to the input of the inverting amplifier 110 at node 108 via a feedback resistor R1 120 that is connected between the two nodes 118 and 108.
By means of suitable choice of the feedback component values of the feedback network 114, the amplifier 100 is forced to self-oscillate at a defined frequency which is typically in the range of 300 kHz to 500 kHz. This high-frequency carrier signal is pulse-width modulated by the audio input signal Ue such that the output spectrum at audio frequencies, typically up to 20 kHz, becomes nearly identical to the input spectrum at node 102. Because the operational amplifier 110 has very high gain in the audio band it suppresses any error or deviation between both spectra.
The binary, switched PWM signal at node 118 is passed to an inductor 122. A capacitor 124 is connected between ground and the inductor 122 at a node 126. Together the inductor 122 and the capacitor 124 form a passive LC reconstruction filter 128. The passive LC reconstruction filter 128 acts as a low-pass filter to remove the high-frequency PWM carrier signal and reconstruct the audio signal as an amplified audio signal capable of driving a loudspeaker represented by a load resistor RL 130 that is connected between node 126 and ground. The reconstructed amplified audio signal is represented as a voltage URL. As such, the conventional class-D amplifier 100 of FIG. 1 acts as a voltage-controlled voltage source.
As can be seen from FIG. 1, the passive LC reconstruction filter 128 is separated from the feedback path through the feedback resistor R1 120. This is unavoidable because the LC reconstruction filter 128 induces phase lag if placed within the feedback loop of the amplifier 100. The phase lag occurs because the voltage at node 126 at the output of the inductor 122 is dependent upon the derivative of the current change through the inductor 122. As such, a time delay is imposed upon the voltage changes at the load RL 130.
There exists a need to provide a voltage-controlled current source-type class-D transconductance amplifier, rather than a voltage-controlled voltage source-type class-D amplifier. This is particularly important for active multiway loudspeaker applications, where a voice coil is directly driven by the amplifier. Since force to the voice coil is directly proportional to current, membrane motion may be controlled more directly, thereby eliminating nonlinear series impedance elements such as voice coil resistance and inductance.